1. Field of the Invention
This invention relates in general to a reversible temperature-indicating composition, and in particular to such a composition composed as a main ingredient of one or more compounds selected from the ester group having a certain particular range of .DELTA.T value (melting point--clouding point) and characterized by an extremely narrow hysteresis margin in the color density-temperature curve.
2. Description of the Prior Art
The conventional reversible temperature-indicating compositions include those materials utilizing the thermochromatic properties of metallic complex salts such as Ag.sub.2 Hg.sub.2 I.sub.4 and Cu.sub.2 HgI.sub.4. Another traditional example is cholesterol liquid crystal.
However, these materials have been found universally unacceptable in practical application because of the following problems.
(1) Metallic complex salts poses a rather very wide margin of hysteresis, that is, do not provide the decoloring effect in reversal at the precise temperature range that previously developes color. This hysteretically instable fluctuation have disqualified them for suitable application in temperature-indicating instruments such as thermometer.
(2) The metallic complex salts and cholesteric liquid crystal are very limited in selecting available chromatic varieties. In the industrial applications of metallic complex salts, only two versions of reversible color change are available, from yellow to orange and red to black. As for cholesterol liquid crystal, practical use is limited almost to color change reversibly brown to or from green and green to or from blue, due to an optical reaction.
(3) These conventional materials suffer a considerable limitation in processing, for example, liquid crystal display. A cholesterol liquid crystal reversible temperature indicating material requires the incorporation of a black substrate layer to insure proper coloring by optical reaction which adds extra manufacturing cost. Further, since it must be kept in completely sealed condition to protect against undesired effects of moisture and humidity, considerable limitations are imposed on only storage and transport but also practical application. Metallic complex salts possess a grave limitation on preparing the matrix through which the material is uniformly distributed. PG,4